1. Field of the Invention
The present invention relates to a method and device for sensing a magnetic field by using magnetoresistive properties present in a superconductive material having grain boundaries therein.
2. Description of the Prior Art
Conventionally, a magnetic sensor which utilizes the Hall effect or magneto-resistive effect in a semiconductor or a magnetic sensor which utilizes the magneto-resistive effect in a magnetic material is widely used for sensing or measuring a magnetic field. The former sensor has a sensitivity capable of sensing a magnetic field of about 10.sup.-2 gauss and the latter one has a sensitivity capable of sensing a magnetic field of about 10.sup.-3 gauss.
However, these conventional magnetic sensors have various disadvantages as follows.
The conventional sensors have relatively large specific resistances R.sub.0 even when no magnetic field is applied to them.
Each variation ratio of resistance to the magnetic field is represented by a parabolic curve having a small coefficient, as shown in FIG. 45 qualitatively.
Since a gain .DELTA.R in the resistance is increased proportional to the square of the magnetic flux density B of an applied magnetic field, the gain to a weak magnetic field of, for example, several tens gauss is very small and, therefore, a ratio of the gain .DELTA.R to the proper resistance R.sub.0 (.DELTA.R/R.sub.0) is an order of 1% at the most.
On the contrary, a magnetic sensor with use of the SQUID (Superconductive Quantum Interference Device) which utilizes the Josephson junction is known to have a very high sensitivity capable of sensing a very weak magnetic field of about 10.sup.-10 gauss. Structures of tunnel junction, point contact and micro bridge have been known as the Josephson junction.
However, the magnetic sensor of this type has a quite delicate structure in manufacturing and requires a complicated operation to use it. Namely, it is not practical for general use, although it has a very high sensitivity.